Objectives: To evaluate the effectiveness of magnesium sulfate as an adjunct to standard therapy for moderate to severe status asthmaticus in hospitalized pediatric patients. Hypothesis: Intravenous magnesium sulfate as an adjunct to standard asthma therapy improves objective measures of expiratory flow and modified pulmonary index scores over standard asthma therapy alone. Study design: Randomized, double blind, placebo controlled trial. Patients: 150 children, aged 5-18 years, with status asthmaticus with PEFR <80% predicted and/or MPIS score >6 admitted to the inpatient unit or ICU after lack of significant improvement with initial acute asthma management. Procedures: After obtaining informed consent from the patients' families and assent from the patients where appropriate, patients will be randomized to receive either magnesium 50 mg/kg (maximum 1 g) or placebo IV every 6 hours. Outcome measures: PEFR and MPIS will be measured at the start of the dosing, 30 and 60 minutes after the initial dose and every 8 hours during therapy. Serum magnesium levels will be checked prior to dosing and daily thereafter. Concomitant therapies, rate of transfer to the ICU, length of ICU stay and length of hospitalization will obtained from the medical record. Analysis: Changes in the PEFR and MPIS scores over time will be compared between the 2 groups using repeated measures analysis of variance. The proportion of patients with limited restriction to expiratory flow (normal PEFR's) and 'normal' MPIS scores at the end of the treatment period will be compared using chi-square or Fisher's exact test.